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        /*
            关键字 var
        */
        var a;
        a = 10;

        // console.log(a)
        /*
        
             =  赋值
        */

        var a = 10;
        // var a = 20;  如果说想要给已经声明的变量赋值，不需要再次声明
        a = 20;


        var b;
        console.log(b)
        /*
            如果只声明，而没有赋值，那么默认为undefined !!!!
        */
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